Optical fiber laser architecture with partitioned pump and signal coupling

ABSTRACT

As kilowatt class fiber laser and amplifier systems become more in demand, there are ongoing efforts to improve optical fiber laser and amplifier designs to maximize efficiency and further increase the capacity of these high-energy optical fiber lasers and amplifiers. The present disclosure provides a fiber laser or amplifier system configured to efficiently and conveniently generate and couple high numerical aperture and high-energy pump light into a fiber laser or amplifier system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application incorporates by reference the following patentapplications as if they were expressly set forth herein in theirentireties:

CROSS-REFERENCE TO RELATED APPLICATIONS

U.S. patent application Ser. No. 14/601,888, having the title “A Pumpand Signal Combiner for High Numerical Aperture Use,” filed concurrentlywith this application;

U.S. patent application Ser. No. 14/601,918, having the title “AFat-Fiber Adapter for Pump Light,” filed concurrently with thisapplication; and

U.S. patent application Ser. No. 14/601,938, having the title “An AllPump Combiner with Cladless Inputs,” filed concurrently with thisapplication.

BACKGROUND

1. Technical Field

The present disclosure relates generally to fiber-based gain systems andfiber laser amplifiers.

2. Description of the Related Art

As kilowatt-class fiber-based gain systems (e.g., fiber lasers, fiberamplifiers, etc.) become more in demand, there are ongoing efforts toimprove these designs to maximize efficiency and further increase thepower capacity of these high-energy optical-fiber-based gain systems.

SUMMARY

In one embodiment, the disclosed gain system utilizes a novelnon-tapered pump and signal combiner, which allows for coupling ofpartitioned high numerical aperture (NA) pump light outside a lasingcavity. Thus, the novel architecture allows for improved performance ascompared with conventional gain systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 shows a configuration of a gain-doped optical fiber laser.

FIG. 2A shows a cross section of an input end of a tapered fiber bundlepump-signal combiner.

FIG. 2B shows a cross section of an output end of a tapered fiber bundlepump-signal combiner.

FIG. 3A shows a configuration of a side-pumping scheme.

FIG. 3B shows a cross section of a fused fiber of FIG. 3A.

FIG. 4 shows an embodiment of an optical fiber laser architectureemploying a non-tapered high numerical aperture (NA) partitioned pumpand signal combiner.

FIG. 5A shows one embodiment of a capillary tube.

FIG. 5B shows one embodiment of coating-free sections of optical fibersinserted in the capillary tube of FIG. 5A.

FIG. 5C shows one embodiment of fused signal and pump fibers aftercollapsing the capillary tube of FIG. 5B.

FIG. 5D shows a cross section of the coating free sections of opticalfiber inserted into the capillary tube before and after collapsing ofthe capillary tube.

FIG. 5E shows one embodiment of a high NA pump and signal combiner.

FIG. 6A shows another embodiment of a capillary tube.

FIG. 6B shows one embodiment of cladless pump input fibers inserted intothe capillary tube of FIG. 6A.

FIG. 6C shows one embodiment of an all pump combiner with cladlessinputs.

FIG. 7 shows cross sections of several embodiments of the disclosed allpump combiner with cladless inputs.

FIG. 8A shows another embodiment of a capillary tube.

FIG. 8B shows coating and cladding free sections of coreless opticalfibers inside the capillary tube of FIG. 8A.

FIG. 8C shows one embodiment of a fat-fiber adapter for pump light.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Optical-fiber-based gain systems (e.g., fiber lasers, fiber amplifiers,etc.) are used in many applications. For example, high powered opticalfiber lasers are used in many fields and industries, including manyapplications relating to cutting and welding.

Demands for increased power output and efficiency are ever present inthe field of optical-fiber-based gain systems. In theory, potentialpower output of optical-fiber-based gain systems have increased overtime as a result of improvements in various aspects of fiber materialsand design as well as signal management techniques such as combiningschemes and use of higher order modes. However, due in part tolimitations of conventional fiber gain architecture, the potentialcapacities of optical-fiber-based gain systems have yet to be fullyrealized.

Factors that Affect Optical-Fiber-Based Gain System Design

To more fully appreciate how conventional optical fiber architecturelimits capacities of fiber-based gain systems, attention is directed toFIGS. 1 through 3B, which detail various aspects of conventionalfiber-based gain systems. As an initial matter, it should be noted that,for fiber amplifiers, a primary optical fiber carries the signal foramplification, while, for fiber lasers, the primary optical fibercarries the fundamental laser mode or signal reflected from a grating(in certain configurations). For simplicity, throughout this document,the fundamental mode and the signal for amplification are both denotedas the “signal.”

To begin, FIG. 1 shows a configuration of a gain-doped optical system100. Specifically, FIG. 1 shows an amplifier configuration. However, onehaving skill in the art will understand that the system can beconfigured as a laser. In a typical end-pumping scheme, for a fiberamplifier, a signal from a signal source 101 is carried by a signalfiber 102, while pump light generated from pump sources 103 is carriedby pigtail pump fibers 104. The signal and pump are then combined in apump and signal combiner 105, which, in most gain systems, is a taperedfiber bundle (TFB). The architecture of a TFB undesirably restricts anamount of pump light that can be introduced into a fiber-based gainsystem.

To fully appreciate why the TFB limits the capacity of a fiber-basedgain system, attention is directed to FIGS. 2A and 2B, which showaspects of the TFB in greater detail. FIG. 2A shows a cross section ofan input end of the TFB 105 (FIG. 1). As shown in FIG. 2A, a signalfiber 102 is surrounded by pump fibers 104, which are located radiallyexterior to the signal fiber 102 on the input end of the TFB 105 (FIG.1). Pump fibers 104 are optically coupled to the signal fiber 102.

With the input configuration of a TFB in mind, attention is turned toFIG. 2B, which shows a cross section of an output end of TFB 105. Asshown in FIG. 2B, as the pump fibers 104 and signal fiber 102 arecombined, they are tapered and effectively fused 203 so as to allow forsplicing (splice point at the end of the TFB 105 is shown as 106 inFIG. 1) to a gain-doped optical fiber 108 (FIG. 1). In other words, thecross sectional area of the TFB 105 is larger on the input end (FIG. 2A)as compared to the output end (FIG. 2B). Consequently, as one havingordinary skill will appreciate, the tapering results in changes to anumerical aperture (NA) of the signal propagating from the input end tothe output end of the TFB. Specifically, the NA increases from the inputend, for example from 0.15, to the output end of the TFB, for example to0.48.

Insofar as the output end (FIG. 2B) is coupled to the gain-doped opticalfiber 108, the NA of the output end of the TFB is restricted to the NAof the gain-doped optical fiber 108. Stated differently, an output NA ofpump and signal combiner 105 (FIG. 1) must be equal to or less than theNA of the optical fiber to which it is spliced to avoid adverse loss oflight. Typically, the gain-doped optical fibers used in fiber-based gainsystems have a NA in the range of approximately 0.4 to 0.48, and have adiameter of approximately 125 microns to approximately 400 microns.Thus, the input light and the taper of the TFB must be controlled suchthat the output light has a NA of 0.48 or less and has a diameter equalto the diameter of the gain doped optical fiber 108 to avoid light loss.Another corresponding drawback is that the core of the signal fiber 102also experiences a tapering effect that changes the modal properties ofthe core.

One having ordinary skill in the art will also appreciate that thesedrawbacks limit the diameter of the input side (FIG. 2A) of the TFB. Forexample, if the input diameter is too large, this will result in toogreat of a taper from input to output side, which consequently resultsin output light having a NA greater than 0.48. This in turn results ininefficient pump light coupling, reduced pump efficiency, and reducedpower output. An increase in the number of pump fibers results in alarger input diameter of the TFB and a corresponding increase in taperand output NA. Therefore, the number of pump fibers that can be combinedwith the signal fiber is limited in currently available end-pumpedfiber-based gain systems. Importantly, this restricts the amount of pumppower that can be introduced into a conventional fiber-based gainsystem, which imparts undesirable limitations on output power.

Moreover, the architecture of the pump fibers in a conventional combinerlimits performance of a fiber-based gain system. Conventional pumpfibers 104 (FIG. 2A) have a glass-clad construction. This glass claddingmaintains the pump light within a core of the pump fiber. It is easy toappreciate that the glass cladding increases the diameter of aconventional pump fiber. As previously discussed, the input diameter ofa conventional TFB combiner is limited. Thus, by increasing the diameterof a pump fiber with a glass cladding, the number of pump fibers thatcan be combined is further restricted so as not to exceed a maximumallowable input diameter.

Further, the glass cladding of conventional pump fibers acts in otherways to restrict pump input. As the pump fibers 104 become tapered andthe NA of the propagating signal increases, pump light having a NA thatis greater than the NA of the pump fiber will leak into the cladding.One having ordinary skill will appreciate that this leads to a decreasein energy density of the combined pump light and a decrease inbrightness. Suffice it to say, there are limitations on how much a TFBcan be tapered without adversely affecting the performance of theoptical-fiber-based gain system. Thus, the increase in NA from input(FIG. 2A) to output (FIG. 2B) of the TFB needs to be preciselycontrolled.

With these and other limitations of a conventional pump and signalcombiner in mind, discussion of the adverse limitations of conventionalgain-system architecture continues with FIG. 1. In a conventionalfiber-based gain system 100, a combined pump and signal output, such asthe output end of a TFB (FIG. 2B), is spliced 106 to a high-reflectorfiber Bragg grating (FBG) 107, which is in turn spliced to thegain-doped optical fiber 108. At the other end, the gain-doped opticalfiber 108 is spliced 109 with an output fiber 112. Additionally, thereis an output coupler 110, typically another FBG, located on the outputfiber 112. The combination of the high reflector FBG 107 and the outputcoupler 110 generate a lasing cavity.

Conventional end-pumped optical-fiber-based gain systems such as thatshown in FIG. 1 have an increased potential for fiber failure. As onehaving ordinary skill in the art will appreciate, introduction of pumplight in an end-pumping scheme (FIG. 1) occurs outside the lasingcavity. Stated differently, in a typical end-pumping scheme, the pump isintroduced before the high reflector FBG (shown as 107 in FIG. 1). Sincewriting a FBG on a core of an optical fiber requires stripping of anoptical fiber cladding followed by cladding patching after writing theFBG, one having ordinary skill will appreciate that the patch creates aweak point in the optical fiber. Therefore, conventional end pumpingschemes have an increased potential for fiber failure as pump light,which is typically high-energy, is propagated over the weak point.

Alternative pumping schemes exist that permit pumping the lasing cavitydirectly. However, these schemes are not as robust as end-pumping andtherefore have met with limited success. One such example is known inthe art as side-pumping, which is shown in FIGS. 3A and 3B. Side-pumpingis a technique of optically pumping a laser gain medium, where the pumplight is introduced substantially parallel to the signal (FIG. 3B). FIG.3A shows a configuration of a side-pumping scheme in which a pump fiber308 is fused adjacent to a signal fiber 309. The resulting side-pumpedfiber is then spliced 306 with a gain-doped fiber 307.

Contrasted with end-pumping (FIG. 1), side-pumping makes it relativelyeasy to receive high-NA pump light. However, conventional side pumpinghas several disadvantages, which make end-pumping a more attractivescheme for most applications. The limited use of side-pumping is alsodue in part to difficulties associated with the manufacture of opticalfibers necessary for use in side-pumping. To manufacture these fibers,they are drawn simultaneously from two side-by-side fiber preforms,thereby adding to complexity and expense in the manufacturing process.In sum, although side-pumping schemes may allow for pumping the lasingcavity directly, their aforementioned shortcomings usually outweightheir benefits.

In sum, conventional configurations of optical-fiber-based gain systemsare limited in their ability to produce high-energy output signalsefficiently. Generally, these limitations arise from an inability ofconventional optical-fiber-based gain systems to efficiently couple highNA pump light. Therefore, there exists a need to designoptical-fiber-based gain systems that can efficiently couple high NApump light to efficiently increase output power of these systems.

Example Embodiments Taking into Account Various Design Factors

The embodiments disclosed herein seek to improve pumping efficiency foroptical fiber and amplifier systems by employing a novel fiber laserarchitecture that relies on producing and coupling a high NA pump light(high meaning about 0.40 to about 0.48) with a signal. In oneembodiment, low NA pump light from several pump diodes are combined toproduce a single high NA pump light, which is propagated along afat-fiber (a fiber where all of the introduced pump light is combined).The high NA pump light is then effectively split into multiple pumps,all having the same high NA. These high NA pumps are combined with asignal fiber by a novel non-tapered fiber combiner. In a preferredembodiment, high NA pump light is combined with the signal within alasing cavity (i.e., between reflecting elements (e.g., FBGs)).

The purpose of combining pump light and then splitting the pump light isto provide a mechanism by which pump sources can be partitioned from thegain system itself. This partitioning permits a more convenient way ofconnecting pump sources to the gain system without the inconvenience ofindividually connecting multiple pump sources, one at a time, to thegain system. Details of this advantage, among others, are providedbelow.

The presently disclosed embodiments permit pumping using a high NA pump,which translates directly into increased pump energy and thus, increasedpower output. Further, several embodiments of the non-tapered combinerallow for pump and signal paths to be divided more cleanly andefficiently than can be realized by conventional pump and signalcombiners, thus, increasing the overall efficiency of the opticalfiber-based gain system. Insofar as the high NA pump is generatedindependently of combining the pump and signal, the present embodimentspermit partitioning of a pump source from the rest of theoptical-fiber-based gain system. Partitioning the pump from theoptical-fiber-based gain system offers several advantages overconventional configurations, such as increasing flexibility inmanufacturing, repairing, and upgrading the optical-fiber-based gainsystem.

As described in greater detail herein, using a fiber laser configurationthat permits pumping high NA light results in increased performance ofthe optical fiber-based gain system. With this in mind, reference is nowmade in detail to the description of the embodiments as illustrated inthe drawings. While several embodiments are described in connection withthese drawings, there is no intent to limit the disclosure to theembodiment or embodiments disclosed herein. On the contrary, the intentis to cover all alternatives, modifications, and equivalents.

With the aforementioned design factors in mind, attention is now turnedto FIGS. 4 through 8C, which show various embodiments of the presentdisclosure. Discussion of the disclosed embodiments begins with a closerexamination of a novel optical-fiber-based gain system architecture.

FIG. 4 shows an embodiment of a fiber-based gain architecture employinga non-tapered high NA pump and signal combiner. Recalling from thedescription above, the term “signal” refers to either the fundamentalmode for a fiber laser or the to-be-amplified signal for a fiberamplifier. As shown in FIG. 4, a signal (e.g., from a signal source 400for a fiber laser) is propagated through a signal fiber (in thisparticular embodiment, a single mode fiber 401) into a novel high NApump and signal combiner 402. Located on the fiber 401 is ahigh-reflector FBG 403. Pump light with a low NA of less thanapproximately 0.22 (preferably approximately 0.15) from pump sources 404are combined in a novel all-pump combiner 405 with cladless inputs,which is discussed in greater detail with reference to FIGS. 6A through6C. The all-pump combiner 405 produces a single pump light with a highNA of approximately 0.45 or greater. In other words, the all-pumpcombiner 405 provides a mechanism by which energy from multiple pumpsources 404 are combined to produce a single higher-energy pump source.Thus, by providing a single high-energy pump source, one canmore-conveniently couple pump light to the gain system through a singleconnector, rather than using multiple connectors, each of whichcorresponds to each of the multiple pump sources.

The all pump combiner 405 further comprises an output fiber 407. Thesingle high NA pump light is then transmitted through a novel fat-fiberadapter 406, which is spliced to the output fiber 407 via input fiber408. The fat-fiber adapter 406 effectively splits a large diameter inputfiber 408 (approximately 300 μm or greater), carrying the high NA pumplight, into multiple smaller diameter fibers 409 while maintaining thesame NA. In other words, the NA of the pump is substantially the samebefore and after being effectively split. The split high NA pump iscombined with the signal through the high NA pump and signal combiner402. As noted above, the all-pump combiner 405 produces a single,high-power pump light. However, since the pump light is introduced intothe gain system through the high NA pump and signal combiner 402, whichhas multiple pigtail fibers, the previously combined pump light must besplit so that it can be input through those pigtail fibers. Thispump-splitting function is performed by the fat-fiber adapter 406, whichinterfaces with the high NA pump and signal combiner 402 to introducethe pump light to the gain system.

The high NA pump and signal combiner 402 comprises an output fiber 410,which is spliced at 411 with a gain-doped optical fiber 412. The gaindopant can be any rare-earth dopant, such as Ytterbium (Yb) or Erbium(Er). The gain-doped fiber is spliced at 413 with an output fiber. Thesignal exits the lasing cavity through an output coupler 414, which ispreferably a FBG. One having ordinary skill in the art will appreciatethat alternatives to FBGs may be used such as mirrors, interferometers,and the like.

Importantly, the novel architecture of the disclosed embodiments permitsthe pump to be introduced within the lasing cavity (i.e., between thetwo reflecting elements (e.g., FBG)). In contrast to conventionalfiber-gain-systems, e.g., FIG. 1 where high-power pump is introducedbefore the weak point created by the cladding patch, in the disclosedembodiments, the pump is introduced after the high-reflector FBG 403(i.e., after the weak point created by the cladding patch)). Insofar asthe high intensity pump does not cross the weak point in the fiber, pumploss is greatly reduced by the present embodiments, leading to improvedlaser efficiency. It should be appreciated that, although it istechnically possible for conventional gain-systems to introduce the pumplight into the lasing cavity, the drawback of this approach forconventional systems is that any signal loss of the combiner is addedinto the lasing cavity itself.

While not specifically shown in FIG. 4, the fiber-gain-system of FIG. 4can be counter-pumped by placing similar pumping components at theoutput end of the gain fiber, before the output coupler 414.Additionally, it should be appreciated that the counter-pumping schemecan be used in conjunction with the pumping scheme shown in FIG. 4 toproduce a co-pumped architecture.

With the details and advantages of the novel fiber-gain system of FIG. 4in mind attention is turned to FIGS. 5A through 5E, which show variousaspects of one embodiment of the non-tapered high NA pump and signalcombiner 402 (FIG. 4). Generally, embodiments of the non-tapered high NApump and signal combiner 402 overcome the aforementioned limitationsthat exist in conventional TFBs by accepting high NA pump light. Becausethere is no significant change in diameter from an input end to anoutput end of the non-tapered high NA pump and signal combiner 402,there is correspondingly no substantial change in NA from the input endto the output end.

With this general description of the non-tapered high NA pump and signalcombiner 402 in mind, attention is directed to FIGS. 5A through 5E,which shows components of one embodiment of a process to manufacture thedisclosed pump and signal combiner 402. Turning to FIG. 5B, themanufacturing process for the non-tapered high NA pump and signalcombiner comprises removing coatings from one end 505 of a signal fiber503 and one or more pump fibers 504 a-504 b (collectively 504).Preferably, the pump fibers 504 are all-silica fibers with a low-indexcoating, which propagates pump light with a NA of approximately 0.48 orgreater. The coating-free ends of the fibers 503, 504 are inserted intothe tube 500 (FIG. 5A) through the open end 501. Continuing, as shown inFIG. 5C, the tube 500 is collapsed on the coating-free sections 505 ofthe fibers 503, 504, effectively fusing together the fibers 503, 504 andforming a non-tapered fused fiber bundle 506.

The non-tapered fused bundle 506 is then cleaved at a suitable location507. Unlike a TFB, the resulting non-tapered fused bundle 506 has nosignificant change in NA from input end to output end due to the lack ofa taper. Suffice it to say that the disclosed non-tapered pump andsignal combiner can accept high NA pump light while still havingefficient coupling to a gain-doped output fiber, which results inimproved optical fiber-based gain system performance over conventionalsystems.

To more fully appreciate the disclosed non-tapered high NA combiner,attention is directed to FIG. 5D, which shows a cross section of thecoating free sections (505, FIG. 5B) of optical fiber inserted into thetube 500 before and after the collapsing of tube 500. As shown in FIG.5D, any decrease in the cross section of the non-tapered fiber combinerfrom the non-fused input fibers to the fused fiber bundle output isminimal and thus, insignificant, as any space between individual fibersis eliminated as the fibers become fused. For example, a single opticalfiber is typically 125 μm in diameter. Thus, in an embodiment of thehigh NA pump and signal combiner that is configured as 7×1 (six pumpfibers 504 surrounding one signal fiber 503), the input side has across-sectional diameter of approximately 375 μm, or the width of threefibers. Fusion of the signal fiber 503 and pump fibers 504 results inelimination of the space between the fibers and a cross-sectionaldiameter of approximately 330 μm.

Continuing with FIG. 5E, after manufacturing the non-tapered fusedbundle 506, the cleaved end (507, FIG. 5C) of the non-tapered fusedbundle 506 is spliced 509 to an output fiber 510. To maximize efficiencyof the non-tapered high NA pump and signal combiner, the output fiber510 is preferably core and cladding match-spliced to the fused bundle ofthe non-tapered high NA pump and signal combiner.

In sum, the disclosed non-tapered high NA pump and signal combineroffers several advantages over conventional combiners and pumpingschemes. The disclosed non-tapered high NA pump and signal combiner canaccept high NA (greater than approximately 0.48) input pump light. Aswill become more apparent with reference to FIGS. 6A through 7, anability to accept high NA pump light permits introduction of more pumpenergy into the system, which results in improved performance foroptical fiber-based gain systems. Additionally, the non-tapered high NAcombiner does not require use of any special pedestal or other fiber asthe signal fiber in the non-tapered fused bundle 506, thus opening apossibility of using arbitrary low mode area fibers orpolarization-maintaining fibers. Collapsing the tube 500 around signalfiber 503 and pump fibers 504 results in a fused fiber bundle with nearperfect core centering, which permits improved splicing to an outputfiber 510. One having ordinary skill will appreciate that improvedsplicing reduces signal and pump loss, which results in increased systemefficiency and performance. Moreover, the non-tapered high NA pump andsignal combiner forms a distinct component that can be spliced at itsoutput onto a gain fiber, which has advantages for manufacturing andrepair.

Additional aspects of the disclosed embodiments capitalize on theability of cladless inputs to accept high NA pump light. For example, asdisclosed in relation FIGS. 6A through 8C, below, more pump light can beintroduced to an optical fiber-based gain system by combining high NApump light, which is typically not feasible for conventionaloptical-fiber-based gain systems such as those shown with reference toFIGS. 1 through 3B. One having an ordinary skill in the art willappreciate that the ability to combine high NA pump light results inincreased power, which is desirable for many applications.

To more fully appreciate how the disclosed embodiments permit more pumplight to be introduced, attention is directed to FIGS. 6A through 6C,which disclose aspects of a novel all pump combiner (405, FIG. 4) ingreater detail. Generally, embodiments of the all pump combiner allowfor combining low NA pump light from several pump sources, resulting ina single high NA pump. With this in mind, discussion of the novel allpump combiner begins with FIG. 6A, which shows another embodiment of acapillary tube 600, preferably made of low index silica glass.

As shown in FIG. 6B, low NA (less than 0.22) pump input fibers 601 havecoating and/or cladding removed on one end of the fibers, forming thecladless pump input fibers 602. In one embodiment the pump input fibers601 are not doped and have a glass cladding. In other embodiments thepump input fibers 601 are down doped and have an air cladding. Thecladless ends of the input fibers 602 are inserted into the capillarytube 600. The capillary tube 600 is then collapsed on the cladless pumpinput fibers 602, effectively fusing them together. The fused cladlesspump input fibers 602 are cleaved at a suitable location 603 on the endopposite of the coated/cladded portion 601 of the pump input fibers,thereby forming the cladless all-pump combiner.

As shown in FIG. 6C, the cladless all-pump combiner 600 is fusionspliced 604 to a single solid optical fiber 605 (or fat fiber), whichpropagates the combined pump light. Preferably, the fat-fiber comprisesa solid all silica fiber. In some embodiments the fat-fiber may betapered at a ratio of approximately 2.72 to 1 in order to produce highNA light of approximately 0.45 or greater.

As cladding alone can represent up to 40% of the pump fiber diameter,removing the coating and/or cladding prior to combining the pump fibersin the all-pump combiner (FIG. 6B) reduces the outer diameter of eachindividual pump fiber. This outer-diameter reduction, when aggregatedover multiple fibers, permits more pump fibers to be packed into anygiven cross-sectional area than if the coating and/or cladding had notbeen removed. The ability to pack more pump fibers together permitsintroduction of more pump light into the optical fiber-based gainsystem, which correspondingly results in increased power. Therefore, theembodiments disclosed herein permit combining more pump light than canbe realized by conventional fiber-based gain systems. Stateddifferently, the disclosed embodiments of the cladless-input all pumpcombiner permits increasing the number of pump fibers that can becombined, thus allowing for increased pump energy to be introduced intothe system resulting in increased optical fiber-based gain systemperformance.

With this key advantage in mind, attention is directed to FIG. 7, whichshows cross sections of several embodiments of the disclosed all pumpcombiner with cladless inputs. Conventional tapered pump and signalcombiners are typically configured in a 7×1 configuration. In otherwords, six pump input fibers are arranged in a hexagonal pattern arounda single signal fiber, and this close-packed structure is then fused andtapered to produce a single output fiber. As previously discussed,conventional TFBs are limited in their input diameter so as to not havetoo great of a taper, which would result in output light with a NAgreater than 0.48 (FIGS. 2A and 2B). Therefore, typical end-pumpedconfigurations are quite limited to how many pump fibers may be combinedusing a single TFB.

In contrast, as shown in FIG. 7, the disclosed embodiments of theall-pump combiner permit more pump fibers to be combined, whichtranslates directly into increased fiber-based gain system output power,as more pump energy is introduced. A 7×1 configuration of pump andsignal fibers is the tightest configuration possible, but also has thelowest energy potential. The number of input pump fibers can beincreased according to the present embodiments to achieve configurationsof at least 19×1 and 26×1, while still producing a single output pumplight with a NA of 0.48. Preferably, the 7×1 and the 19×1 configurationsare packed in a tight hexagonal configuration. Other embodiments mayhave looser packing structures.

It should be appreciated that the cladless input all-pump combinerallows the pump source to be effectively partitioned from the rest ofthe system, thus increasing flexibility in manufacturing, repairing, andupgrading the optical-fiber-based gain system as compared toconventional configurations. With this in mind, attention is directed toFIGS. 8A through 8C, which show various aspects of a fat fiber adapter,which, in some embodiments, connects the combined pump light from theall pump combiner to an optical-fiber-based gain system.

As shown in FIG. 8A, manufacture of the fat fiber begins with acapillary tube 800 with a flared end 801. Continuing with FIG. 8B, thecoating is removed from ends of coreless optical fibers 802, which arepreferably silica-based fibers. Coreless silica fibers are used in thisembodiment to avoid additional glass that can attenuate lightpropagation through the optical fibers that eventually comprise thefat-fiber adapter. The coating free ends 802 of the coreless opticalfibers are inserted into the capillary tube 800 through the flared end801 such that coated regions of the coreless optical fibers 803 remainoutside of the capillary tube 800. The capillary tube 800 is collapsedon the bundle of coreless optical fibers 802 within the capillary tube800, effectively fusing them. The fused fibers are then cleaved 804 at asuitable place on the fused end of the fibers forming an input end.Capacity of the optical fiber or amplifier system dictates the number ofoutput fibers 803 used in the fat-fiber adapter. For example, for anoptical-fiber-based gain system with a 7×1 pump and signal combiner, sixoutput fibers 803 would stem from the fat fiber adapter. One havingordinary skill in the art will appreciate that other numbers of outputfibers may be used.

Continuing with FIG. 8C, the cleaved end 804 (FIG. 8B) of the fusedbundle 805 is spliced 806 to the fat-fiber 807. Preferably, the fusedbundle 805 is formed to substantially match the input fat-fiber 807. Onehaving an ordinary skill in the art will appreciate that any mismatch inoverlap between the input fat-fiber 807 and the fused bundle 805 willresult in pump light loss where no over lap between the input fat-fiber807 and the fused bundle 805. In some embodiments, the fused bundle 805shape is not substantially round, and there is a gradual transition inshape lengthwise along the fused bundle 805. This gradual transition isbeneficial as it avoids abrupt changes and maintains interstitialspacing between the fibers in the fused bundle 805.

Preferably, the input fat-fiber 807 is optically coupled to the all-pumpcombiner described in FIGS. 6A through 7. As the fat-fiber adapter isconfigured to accept high NA pump light, one with ordinary skill willappreciate that for some embodiments it can also accept any light with aNA of less than 0.48. Therefore, for some embodiments of the fat-fiberadapter, the input fat-fiber 807 comprises a pump fiber from a genericpump source.

The output fibers 803 are preferably connected to the non-tapered highNA pump and signal combiner described in FIGS. 5A through 5D. For otherembodiments, the output fibers may be connected to a conventional pumpand signal combiner, such as a TFB. One having an ordinary skill in theart will appreciate that in embodiments that have the output fibers 803coupled to an optical fiber-based gain system through a conventionalpump and signal combiner, such as a TFB, the input light may be a low NA(approximately less than 0.22, preferably approximately 0.15) to avoidloss of light due to the change in NA resulting from the taper in a TFB.

Although exemplary embodiments have been shown and described, it will beclear to those of ordinary skill in the art that a number of changes,modifications, or alterations to the disclosure as described may bemade.

What is claimed is:
 1. A system, comprising: a gain-doped optical fibercomprising a lasing cavity, the lasing cavity comprising a firstreflecting element and a second reflecting element; a non-tapered pumpand signal combiner optically coupled to the lasing cavity between thefirst reflecting element and the second reflecting element, thenon-tapered pump and signal combiner configured to receive highnumerical aperture (NA) pump light, the non-tapered pump and signalcombiner further configured to receive a signal, the non-tapered pumpand signal combiner further configured to combine the high NA pump lightwith the signal to produce a pumped signal, the non-tapered pump andsignal combiner further configured to propagate the pumped signal; and afat-fiber adapter optically coupled to the non-tapered pump and signalcombiner, the fat-fiber adapter configured to split a high NA input pumplight into multiple high NA pump light inputs.
 2. The system of claim 1,the high NA pump light having a NA that is greater than approximately0.22.
 3. The system of claim 2, the high NA pump light having a NA thatis less than approximately 0.48.
 4. The system of claim 1, the firstreflecting element comprising a fiber Bragg grating (FBG).
 5. The systemof claim 1, the second reflecting element comprising a fiber Bragggrating (FBG).
 6. A system comprising: a gain-doped optical fiber; anon-tapered pump and signal combiner optically coupled to the gain-dopedoptical fiber, the non-tapered pump and signal combiner configured toreceive high numerical aperture (NA) pump light, the non-tapered pumpand signal combiner further configured to receive a signal, thenon-tapered pump and signal combiner further configured to combine thehigh NA pump light with the signal to produce a pumped signal, thenon-tapered pump and signal combiner further configured to propagate thepumped signal; and a fat-fiber adapter optically coupled to thenon-tapered pump and signal combiner, the fat-fiber adapter configuredto split a high NA input pump light into multiple high NA pump lightinputs.
 7. The system of claim 6, the high NA pump light having a NAthat is greater than approximately 0.22.
 8. The system of claim 6, thehigh NA pump light having a NA that is less than approximately 0.48. 9.The system of claim 6, further comprising an input fiber opticallycoupled to an input end of the non-tapered pump and signal combiner, theinput fiber to introduce a signal to the non-tapered pump and signalcombiner.
 10. The system of claim 6, further comprising an output fibercoupled to an output end of the gain-doped optical fiber.
 11. The systemof claim 6, further comprising: a first reflecting element opticallycoupled to the gain-doped optical fiber; a second reflecting elementoptically coupled to the gain-doped optical fiber; and a lasing cavitylocated between the first reflecting element and the second reflectingelement.
 12. The system of claim 11, the non-tapered pump and signalcombiner being optically coupled to the lasing cavity between the firstreflecting element and the second reflecting element.
 13. The system ofclaim 11, the first reflecting element comprising a fiber Bragg grating(FBG).
 14. The system of claim 11, the second reflecting elementcomprising a fiber Bragg grating (FBG).
 15. The system of claim 6, thegain-doped optical fiber comprising a rare earth dopant.
 16. The systemof claim 6, further comprising a pump combiner optically coupled to thefat-fiber adapter, the pump combiner to combine low NA pump lightcladless inputs into a high NA output.
 17. The system of claim 16, thelow NA pump light having a NA of less then approximately 0.22.
 18. In asystem comprising a gain-doped optical fiber, a non-tapered pump andsignal combiner, and a fat-fiber adapter, a method comprising: splittinga high numerical aperture (NA) input pump light into multiple high NApump light inputs using the fat-fiber adapter; receiving high NA pumplight inputs at the non-tapered pump and signal combiner; receiving asignal at the non-tapered pump and signal combiner; combining the highNA pump light with the signal at the non-tapered pump and signalcombiner to produce a pumped signal; and propagating the pumped signalin the gain-doped optical fiber.